(1) Cell:cell interactions between antibody-coated spleen cells and P388D1 cells have been examined using a novel flow microfluorometric technique. The mechanism of FcGammaR-mediated aggregation has been studied. (2) The distributions of FcGammaR have been studied in mouse T and B cells and in macrophages by dual parameter flow cytometry. Distributions in normal mice differ significantly from those in mice infected with different pathogenic agents. (3) A new type of effector cell has been generated by treating ADCC effector cells with anti-FcGammaR antibody cross linked to anti-target cell antibodies. (4) The appearance of neo MHC determinants has been detected on TNP modified spleen cells, using anti-MHC monoclonal antibodies. These results correlate with cross reactivities seen in CML reactions.